Documentation/devicetree/bindings/writing-schema.rst - linux - Git at Google

 .. SPDX-License-Identifier: GPL-2.0

 Writing Devicetree Bindings in json-schema
 ==========================================

 Devicetree bindings are written using json-schema vocabulary. Schema files are
 written in a JSON-compatible subset of YAML. YAML is used instead of JSON as it
 is considered more human readable and has some advantages such as allowing
 comments (Prefixed with '#').

 Also see :ref:`example-schema`.

 Schema Contents
 ---------------

 Each schema doc is a structured json-schema which is defined by a set of
 top-level properties. Generally, there is one binding defined per file. The
 top-level json-schema properties used are:

 $id
   A json-schema unique identifier string. The string must be a valid
   URI typically containing the binding's filename and path. For DT schema, it must
   begin with "http://devicetree.org/schemas/". The URL is used in constructing
   references to other files specified in schema "$ref" properties. A $ref value
   with a leading '/' will have the hostname prepended. A $ref value with only a
   relative path or filename will be prepended with the hostname and path
   components of the current schema file's '$id' value. A URL is used even for
   local files, but there may not actually be files present at those locations.

 $schema
   Indicates the meta-schema the schema file adheres to.

 title
   A one-line description on the contents of the binding schema.

 maintainers
   A DT specific property. Contains a list of email address(es)
   for maintainers of this binding.

 description
   Optional. A multi-line text block containing any detailed
   information about this binding. It should contain things such as what the block
   or device does, standards the device conforms to, and links to datasheets for
   more information.

 select
   Optional. A json-schema used to match nodes for applying the
   schema. By default, without 'select', nodes are matched against their possible
   compatible-string values or node name. Most bindings should not need select.

 allOf
   Optional. A list of other schemas to include. This is used to
   include other schemas the binding conforms to. This may be schemas for a
   particular class of devices such as I2C or SPI controllers.

 properties
   A set of sub-schema defining all the DT properties for the
   binding. The exact schema syntax depends on whether properties are known,
   common properties (e.g. 'interrupts') or are binding/vendor-specific
   properties.

 A property can also define a child DT node with child properties defined
 under it.

 For more details on properties sections, see 'Property Schema' section.

 patternProperties
   Optional. Similar to 'properties', but names are regex.

 required
   A list of DT properties from the 'properties' section that
   must always be present.

 examples
   Optional. A list of one or more DTS hunks implementing the
   binding. Note: YAML doesn't allow leading tabs, so spaces must be used instead.

 Unless noted otherwise, all properties are required.

 Property Schema
 ---------------

 The 'properties' section of the schema contains all the DT properties for a
 binding. Each property contains a set of constraints using json-schema
 vocabulary for that property. The properties schemas are what are used for
 validation of DT files.

 For common properties, only additional constraints not covered by the common,
 binding schema need to be defined such as how many values are valid or what
 possible values are valid.

 Vendor-specific properties will typically need more detailed schema. With the
 exception of boolean properties, they should have a reference to a type in
 schemas/types.yaml. A "description" property is always required.

 The Devicetree schemas don't exactly match the YAML-encoded DT data produced by
 dtc. They are simplified to make them more compact and avoid a bunch of
 boilerplate. The tools process the schema files to produce the final schema for
 validation. There are currently 2 transformations the tools perform.

 The default for arrays in json-schema is they are variable-sized and allow more
 entries than explicitly defined. This can be restricted by defining 'minItems',
 'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed
 size is desired in most cases, so these properties are added based on the
 number of entries in an 'items' list.

 The YAML Devicetree format also makes all string values an array and scalar
 values a matrix (in order to define groupings) even when only a single value
 is present. Single entries in schemas are fixed up to match this encoding.

 Coding style
 ------------

 Use YAML coding style (two-space indentation). For DTS examples in the schema,
 preferred is four-space indentation.

 Testing
 -------

 Dependencies
 ~~~~~~~~~~~~

 The DT schema project must be installed in order to validate the DT schema
 binding documents and validate DTS files using the DT schema. The DT schema
 project can be installed with pip::

     pip3 install dtschema

 Note that 'dtschema' installation requires 'swig' and Python development files
 installed first. On Debian/Ubuntu systems::

     apt install swig python3-dev

 Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be
 installed. Ensure they are in your PATH (~/.local/bin by default).

 Recommended is also to install yamllint (used by dtschema when present).

 Running checks
 ~~~~~~~~~~~~~~

 The DT schema binding documents must be validated using the meta-schema (the
 schema for the schema) to ensure they are both valid json-schema and valid
 binding schema. All of the DT binding documents can be validated using the
 ``dt_binding_check`` target::

     make dt_binding_check

 In order to perform validation of DT source files, use the ``dtbs_check`` target::

     make dtbs_check

 Note that ``dtbs_check`` will skip any binding schema files with errors. It is
 necessary to use ``dt_binding_check`` to get all the validation errors in the
 binding schema files.

 It is possible to run both in a single command::

     make dt_binding_check dtbs_check

 It is also possible to run checks with a subset of matching schema files by
 setting the ``DT_SCHEMA_FILES`` variable to a specific schema file or pattern.

 ::

     make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml
     make dt_binding_check DT_SCHEMA_FILES=/gpio/
     make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml


 json-schema Resources
 ---------------------


 `JSON-Schema Specifications <http://json-schema.org/>`_

 `Using JSON Schema Book <http://usingjsonschema.com/>`_

 .. _example-schema:

 Annotated Example Schema
 ------------------------

 Also available as a separate file: :download:`example-schema.yaml`

 .. literalinclude:: example-schema.yaml
	.. SPDX-License-Identifier: GPL-2.0

	Writing Devicetree Bindings in json-schema
	==========================================

	Devicetree bindings are written using json-schema vocabulary. Schema files are
	written in a JSON-compatible subset of YAML. YAML is used instead of JSON as it
	is considered more human readable and has some advantages such as allowing
	comments (Prefixed with '#').

	Also see :ref:`example-schema`.

	Schema Contents
	---------------

	Each schema doc is a structured json-schema which is defined by a set of
	top-level properties. Generally, there is one binding defined per file. The
	top-level json-schema properties used are:

	$id
	A json-schema unique identifier string. The string must be a valid
	URI typically containing the binding's filename and path. For DT schema, it must
	begin with "http://devicetree.org/schemas/". The URL is used in constructing
	references to other files specified in schema "$ref" properties. A $ref value
	with a leading '/' will have the hostname prepended. A $ref value with only a
	relative path or filename will be prepended with the hostname and path
	components of the current schema file's '$id' value. A URL is used even for
	local files, but there may not actually be files present at those locations.

	$schema
	Indicates the meta-schema the schema file adheres to.

	title
	A one-line description on the contents of the binding schema.

	maintainers
	A DT specific property. Contains a list of email address(es)
	for maintainers of this binding.

	description
	Optional. A multi-line text block containing any detailed
	information about this binding. It should contain things such as what the block
	or device does, standards the device conforms to, and links to datasheets for
	more information.

	select
	Optional. A json-schema used to match nodes for applying the
	schema. By default, without 'select', nodes are matched against their possible
	compatible-string values or node name. Most bindings should not need select.

	allOf
	Optional. A list of other schemas to include. This is used to
	include other schemas the binding conforms to. This may be schemas for a
	particular class of devices such as I2C or SPI controllers.

	properties
	A set of sub-schema defining all the DT properties for the
	binding. The exact schema syntax depends on whether properties are known,
	common properties (e.g. 'interrupts') or are binding/vendor-specific
	properties.

	A property can also define a child DT node with child properties defined
	under it.

	For more details on properties sections, see 'Property Schema' section.

	patternProperties
	Optional. Similar to 'properties', but names are regex.

	required
	A list of DT properties from the 'properties' section that
	must always be present.

	examples
	Optional. A list of one or more DTS hunks implementing the
	binding. Note: YAML doesn't allow leading tabs, so spaces must be used instead.

	Unless noted otherwise, all properties are required.

	Property Schema
	---------------

	The 'properties' section of the schema contains all the DT properties for a
	binding. Each property contains a set of constraints using json-schema
	vocabulary for that property. The properties schemas are what are used for
	validation of DT files.

	For common properties, only additional constraints not covered by the common,
	binding schema need to be defined such as how many values are valid or what
	possible values are valid.

	Vendor-specific properties will typically need more detailed schema. With the
	exception of boolean properties, they should have a reference to a type in
	schemas/types.yaml. A "description" property is always required.

	The Devicetree schemas don't exactly match the YAML-encoded DT data produced by
	dtc. They are simplified to make them more compact and avoid a bunch of
	boilerplate. The tools process the schema files to produce the final schema for
	validation. There are currently 2 transformations the tools perform.

	The default for arrays in json-schema is they are variable-sized and allow more
	entries than explicitly defined. This can be restricted by defining 'minItems',
	'maxItems', and 'additionalItems'. However, for DeviceTree Schemas, a fixed
	size is desired in most cases, so these properties are added based on the
	number of entries in an 'items' list.

	The YAML Devicetree format also makes all string values an array and scalar
	values a matrix (in order to define groupings) even when only a single value
	is present. Single entries in schemas are fixed up to match this encoding.

	Coding style
	------------

	Use YAML coding style (two-space indentation). For DTS examples in the schema,
	preferred is four-space indentation.

	Testing
	-------

	Dependencies
	~~~~~~~~~~~~

	The DT schema project must be installed in order to validate the DT schema
	binding documents and validate DTS files using the DT schema. The DT schema
	project can be installed with pip::

	pip3 install dtschema

	Note that 'dtschema' installation requires 'swig' and Python development files
	installed first. On Debian/Ubuntu systems::

	apt install swig python3-dev

	Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be
	installed. Ensure they are in your PATH (~/.local/bin by default).

	Recommended is also to install yamllint (used by dtschema when present).

	Running checks
	~~~~~~~~~~~~~~

	The DT schema binding documents must be validated using the meta-schema (the
	schema for the schema) to ensure they are both valid json-schema and valid
	binding schema. All of the DT binding documents can be validated using the
	``dt_binding_check`` target::

	make dt_binding_check

	In order to perform validation of DT source files, use the ``dtbs_check`` target::

	make dtbs_check

	Note that ``dtbs_check`` will skip any binding schema files with errors. It is
	necessary to use ``dt_binding_check`` to get all the validation errors in the
	binding schema files.

	It is possible to run both in a single command::

	make dt_binding_check dtbs_check

	It is also possible to run checks with a subset of matching schema files by
	setting the ``DT_SCHEMA_FILES`` variable to a specific schema file or pattern.

	::

	make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml
	make dt_binding_check DT_SCHEMA_FILES=/gpio/
	make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml


	json-schema Resources
	---------------------


	`JSON-Schema Specifications <http://json-schema.org/>`_

	`Using JSON Schema Book <http://usingjsonschema.com/>`_

	.. _example-schema:

	Annotated Example Schema
	------------------------

	Also available as a separate file: :download:`example-schema.yaml`

	.. literalinclude:: example-schema.yaml